This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Based on our initial success with continuously variable coupling at 95 GHz, we have extended the concept to our high field spectrometer at 170 and 240 GHz. Variable coupling is crucial to the success of our experiments as the achievable signal to noise depends strongly on how closely we can achieve critical coupling. Our initial attempts, where the coupling structure was simply scaled from 95 to 170 and 240 GHz using dimensional analysis, gave a usable performance level, but not optimum as we were able to verify experimentally. The optimum structure at higher frequencies seems to be asymmetric, based on our empirical tests. We are currently making a systematic study of these asymmetric structures at higher frequencies in order to further explore the range of couplings that are available. We are also using our physical optics package to gain insights into the performance of these novel structures and directions for future improvements. The empirical optimization procedure we have been using so far has led to approximately one order of magnitude improvement in signal to noise, with a reduction in spectral acquisition time of two orders of magnitude for comparable signal to noise ratio. These improvements will allow us to design experiments for studying spin-labeled cells, whose short life-times require rapid spectral acquisition.